1. Field of the Invention
The present invention relates to a digital convergence correction device and to a display device comprising the digital convergence correction device.
2. Description of the Related Art
Generally, in digital convergence correction device, a plurality of points on a CRT screen are established to be convergence correction points, and a convergence correction amount in each convergence correction point is preliminarily obtained and stored in a memory as correction data. Then, the correction data are read out from the memory synchronously with screen scanning, and the correction data read out are converted to an analog signal. Then, a convergence correction signal is generated by interpolating (smoothing) the analog signal with a low-pass filter, and a convergence yoke coil (CY coil) of the CRT is driven.
The convergence correction amount is a value determined depending on a position on the CRT screen. That is, the convergence correction amount corresponds to (is associated with) a position on the CRT screen in a relation of 1 to 1. Therefore, the plurality of convergence correction points are predetermined so as to correspond to the position on the CRT screen in a relation of 1 to 1, and with respect to each convergence correction point, the preliminarily obtained convergence correction amount is stored in the memory as correction data.
Accordingly, if a relation between an effective area of picture which is an effective display region on the CRT screen and a screen raster which is a scanning region on the CRT screen varies, there arises a problem that the correction data read out synchronously with the screen scanning fail to correspond to the convergence correction points on the CRT screen.
For example, such a problem arises in the event that the screen raster is enlarged in the direction of over-scan otherwise reduced in the direction of under-scan, or in the event that aspect ratio of the screen raster varies, or in the event that the screen raster is shifted vertically or horizontally within the effective area of picture. Particularly, in the display device capable of inputting an picture signal of different scanning frequency, a relation between the screen raster and the effective area of picture of CRT may vary due to variation in deflecting frequency. In such a case, it is impossible to perform the convergence correction appropriately.
The Japanese Laid-Open Patent Publication (unexamined) No. 33791/1985 discloses a digital convergence correction device capable of performing a convergence correction even if the relation between the screen raster and the effective area of picture of CRT varies. This convergence correction device comprises a memory for storing correction data for each scanning frequency and a low-pass filter for each scanning frequency, and is used by detecting a scanning frequency of picture signal inputted and by selecting a memory and a low-pass filter corresponding to the detected scanning frequency.
However, to perform the convergence correction appropriately, it is necessary to provide a large number of convergence correction points on the CRT screen. To store the correction data for each convergence correction point, a memory of large capacity is required. Accordingly, there is a problem that the convergence correction device provided with such a memory for each scanning frequency is expensive, and the display device is also expensive.
There is a further problem that as a result of providing a low-pass filter corresponding to each scanning frequency, circuitry is complicated and, therefore, the convergence correction device is expensive and the display device is further expensive.
Moreover, in such a conventional convergence correction device, as the relation between the screen raster and the effective area of picture is specified by a scanning frequency, if the relation between the screen raster and the effective area of picture of CRT is different when a picture signal of identical scanning frequency is inputted, there is a further problem that any convergence correction cannot be performed appropriately.
On the other hand, the Japanese Laid-Open Patent Publication (unexamined) No. 20809/1995 discloses a digital convergence correction device of relatively reasonable cost which performs a convergence correction for a picture signal of different scanning frequency. In this convergence correction device, without synchronization with the screen scanning, correction data are read out from a memory with a certain period. Accordingly, even if any picture signal of different scanning frequency is inputted, time interval for reading out the correction data can be constant, and without changing any low-pass filter, it is possible to correspond to the picture signal of different frequency. That is, as this convergence correction device comprises only one low-pass filter, it is possible to provide a convergence correction device of relatively reasonable cost capable of corresponding to different scanning frequency. However, the mentioned problem that a memory of large capacity is required still remains unsolved. Moreover, in the event that the relation between the screen raster and the effective area of picture is different, it is impossible to perform any convergence correction appropriately.